Ablation therapy uses heat to kill undesirable tissue. Energy from various sources, often radiofrequency (RF) electrical energy, but also microwave, ultrasound, or laser energy, is used to heat tissue to a therapeutic temperature and thereby kill the tissue. This therapy can be used to treat various small tumors, including malignant tumors such as liver cancer or benign tumors such as uterine fibroids, by overheating and killing the tumorous tissue. It can also be used to ablate arrhythmogenic heart tissue that causes arrhythmias, such as atrial flutter. While ablation therapy is often used to treat small amounts of tissue such as those mentioned above, conducting the therapy for larger tumors, or for other conditions that require treating large volumes of tissue, remains difficult.
One prior art solution for ablating larger volumes of tissue has been the use of microwave energy in place of RF electrical energy. Microwave energy can be better suited for use in large treatment volumes because its energy can penetrate farther into tissue than RF energy. For example, microwave energy at 915 MHz can penetrate approximately 4 cm (in theory) into tissue, as compared to RF energy that is typically dissipated beyond about 1 cm from the electrodes that deliver the energy. Therefore, microwave energy can directly heat a greater volume of tissue than RF energy.
Microwave energy can have additional advantages over other energy sources as well. For example, microwaves propagate through all types of tissues and non-metallic materials, including charred and desiccated tissues that can be created during the ablative process. In contrast, when tissue becomes charred or desiccated by RF ablation the impedance of the tissue rises dramatically, making it difficult to pass further current through the tissue and effectively terminating the therapy. Still further, microwaves can deliver greater levels of direct heating energy as compared to other ablation energies, which can be advantageous when ablation is conducted in organs with high blood perfusion or near sources of blood flow, such as veins or arteries, that can draw heat away from a target tissue volume.
However, microwave energy does have drawbacks as well. For example, while the depth of treatment may extend farther into tissue than with RF energy, it is still limited. As mentioned above, in theory the field is typically dissipated at about 4 cm from the microwave antenna. In practice, the cylindrical configuration of microwave antenna exacerbate the radial spreading of that energy into tissue being treated, resulting in dissipation within 2 cm of the cylindrical antenna. Treating larger volumes of tissue therefore requires repositioning the antenna or using multi-antenna arrays. Further, the dynamics of the microwave field can become complex when multi-antenna arrays are used. As a result, therapy procedure times and costs can be significantly increased due to either repositioning a single probe several times or setting up a multi-antenna array.
In addition, the microwave energy deposition field is static and is defined by the electromagnetic properties of the surrounding tissues and the geometry of the antenna itself. Within the heating field there are volumes of tissue that are heated more or less than others (e.g., similar to reheating food in a microwave oven). This can lead to undesirable therapies, as some tissue can be heated to a dangerous level by a strong microwave field (e.g., becoming superheated and explosively converting to steam), while other tissue can be heated to a sub-therapeutic temperature by a weaker microwave field. Thermal energy does flow from the volumes of tissue that are heated by strong microwave fields to those heated by weaker microwave fields by thermal conduction, but this is a slow and inefficient process in tissue, and does not address the safety concern created by superheating portions of a target volume of tissue.
Moreover, microwave energy can overheat the antenna used for therapy application and its associated cabling used to transfer power to the antenna from a generator. This internal heating must be countered by limiting energy transmission or by actively cooling the components of the microwave ablation system to prevent undesired thermal damage to tissues in contact with the antenna or the cabling extending to the antenna. Prior art methods for actively cooling microwave antennas include circulating a fluid or cryogenic gas along the length of the cable and even through the antenna itself, but these closed-loop systems can result in large diameter devices, e.g., due to the need for delivery and return lumens.
Accordingly, there is a need for improved devices and methods for conducting ablation using microwave antennas. In particular, there is a need for microwave ablation devices and methods that can deliver therapeutic doses of thermal energy to large volumes of tissue and address cooling issues commonly encountered with microwave antennas.